The invention relates to a nuclear magnetic resonance coil configuration having at least one flat or cylindrical coil, through which current flows in operation, which coil generates a high-frequency magnetic B1 field at the location of a sample which is oriented parallel to an x-axis, and which for the purpose of connection to a tuning network is connected to at least two electrical feed lines, through which in-phase currents flow in operation, and which generate a high-frequency magnetic B2 field in the sample, the orientation of which encloses an angle α with the direction of the B1 field.
Such a configuration is disclosed in U.S. Pat. No. 7,397,246 B2.
Multi-turn saddle coils and their feed lines are shown in U.S. Pat. No. 7,397,246 B2. U.S. Pat. No. 6,812,703 B2 shows a resonator with feed lines. Coils and resonators with feed lines are also illustrated in FIGS. 8.22, 8.26, 8.33, 8.36, 8.61, 8.68, 8.76 in “NMR Probeheads for Biophysical and Biomedical Experiments: Theoretical Principles & Practical Guidelines;” Joël Mispelter, Mihaela Lupu, André Briguet; Imperial College Press, 2006; ISBN 1860946372
Probe heads, in which, as a rule, coil configurations which consist of at least one saddle coil, at least one birdcage, Alderman-Grant or comparable resonators, are fitted, are used in the high-resolution NMR of liquids. Planar coils or resonators are also used, particularly in the field of probe heads for micro samples and also for probe heads which use high-temperature superconducting material for the transmitting and/or receiving coil configurations. In the following, coils and resonators are not explicitly differentiated, i.e. when coils 18 are being discussed, this refers to coils and/or resonators. There is no differentiation between coils on a flat or cylindrical substrate either.
These coils 18 are connected by means of feed lines 11 to a network, by means of which the coils can be tuned to one or more resonant frequencies which serve as transmitting and receiving frequencies when the probe head is in operation. FIGS. 1a, 1d, 1 e and 1f show by way of example single and two-turn saddle coil configurations, a birdcage resonator and a double single-turn series coil configuration, in each case including feed lines 11, in a developed view.
In FIG. 1a, the intersection of the positive direction of the xz-plane with the development is denoted by 0° and that with the negative direction of the xz-plane by 180°. The yx-plane intersects the coil in the center and is shown schematically as section plane A-A′. The equivalent positions with regard to the coordinate system apply in the other drawings even when they are not explicitly shown or designated.
Coil configurations of this kind can be equipped with two (FIGS. 1e and 1f), with three (FIG. 1d) or with four feed lines (FIG. 1a) and the coil halves can be connected in series or parallel. The connection can be made within the coil (FIGS. 1d to 1f) or outside (FIGS. 1a and 1 d). A birdcage resonator can be designed as a high pass, low pass or band pass birdcage; the special embodiment in FIG. 1e serves merely to illustrate a resonator. A saddle coil with integrated capacitance between the conductors which is operated in self-resonant mode also constitutes a resonator. As a rule, resonators are connected to the network by means of just two feed lines as, in operation, the current is limited substantially within the resonator and no advantages are to be expected by using a multiplicity of feed lines. Particularly for resonators, it is also common to couple them inductively, wherein the coupling inductance becomes part of the resonator. Here, the feed lines to such a coupling coil are understood to be coil feed lines.
The saddle coils used as well as resonators are normally cylindrical and consist of “bar elements” 12, i.e. elements which are oriented substantially parallel to the cylinder axis, and “ring elements” 13, i.e. elements which are oriented substantially orthogonal to the cylinder axis.
Generally, the samples 16 are also cylindrical or ellipsoidal and have a cylinder or main axis. The axes of the coils and samples coincide with the z-axis of a coordinate system. The field direction of the static magnetic field is in general oriented parallel, orthogonal or at the so-called magic angle of approx. 54° to this axis.
In general, both the samples 16 and the coils are designed to be at least substantially cylindrical, wherein the conductor elements of the coils (both bar elements 12 and ring elements 13) lie on one or two radii. Only any possibly included crossing points 14 that may be included deviate from these radii. The coils can also include support elements 18 made of dielectric material. Connections between the radii can be achieved by capacitively acting elements (distributed capacitances by means of integrated plate capacitors, e.g. formed by overlapping conductor elements and the dielectric support material 18, or localized capacitances realized by discrete elements, i.e. soldered-in capacitors) or galvanic connections.
Bar elements and ring elements can also be differentiated in the case of flat (planar or biplanar) coils; in doing so, it should always be assumed that the bar elements are oriented substantially parallel to the z-axis and the ring elements substantially orthogonal thereto.
The feed lines can be connected either to the ring elements (FIG. 1a) or to the bar elements (FIG. 1e). It is also possible to connect one feed line to a ring element and a second to a bar element (FIGS. 1d and 1f).
In operation, the coils generate a high-frequency (HF) magnetic (B) field which is designated here as a B1 field. A B1 field is to be understood only as the part of the high-frequency magnetic field which is generated by the current through the conductors (and through any integrated capacitors) of the coil, but not by the currents through other conductors, such as the feed lines to the coil for example. In doing so, the frequency is tuned such that it corresponds substantially to the resonant frequency of the nuclear species to be detected in the given static magnetic field B0.
This B1 field can either be polarized linearly or circularly. In case of a circularly polarized field, at least two pairs of feed lines are required for excitation. The latter are generally designed orthogonally to one another and excite the two modes of the coil that are tuned to the same frequency. These pairs of feed lines can also be formed such that they have a common feed line, i.e. they consist of just three feed lines.
Each of these modes can be described by a linearly polarized field. This linearly polarized field has a preference direction and at least substantially a symmetry plane, to which the field in the measuring volume lies parallel. For a linearly polarized coil, this plane shall coincide with the xz-plane.
For circularly polarized coils, the plane defined by the B1 field of the first linearly polarized mode shall coincide with the xz-plane. The plane defined by the second mode then generally coincides with the yz-plane. All feed lines that are used to excite a linearly polarized mode or coil with just one mode have in-phase currents. The currents in feed lines for a degenerate second linearly polarized mode of a coil operated in quadrature have a phase shift of approximately +90° or −90° to excite the first linearly polarized mode. A coil operated in quadrature can be considered as a combination of two linearly operated coils, wherein both coils have common conductor elements. Only linearly polarized coils are considered in the following, and consequently we will also only consider feed lines to a coil or resonator which all have the same phase. Feed lines to different coils (and therefore also different modes of one and the same coil) are explicitly not considered. Currents which flow “upwards” in one feed line and “downwards” in a second are to be considered as being in-phase.
A coil consists of a central region 21, in which the highest B1 field amplitudes are achieved, and a top 22 and bottom 23 edge region, in which the B1 field decreases and its magnitude tends to 0 with or without a zero crossing. If a zero crossing is present, the coils have a local maximum 24 in the edge region. The bottom region is understood to mean that region in the vicinity of which the feed lines are fitted or in the direction of which the feed lines are fed regardless of how this direction is effectively oriented in space. The top region is understood to be the edge region without feed lines. Coils also exist in which feed lines are fitted on both sides. In this case, this coil has only bottom regions and has no top edge region. In the latter case, the positive direction of the z-axis can be chosen at will, otherwise the positive z-direction must point in the direction of the top edge regions.
In the known prior art, the feed lines are normally fitted in each case as pairs of two feed lines at the same end of the coil and then fed downwards substantially parallel to the cylinder axis. The pairs of feed lines are usually either designed such that they lie symmetrically with respect to the xz-plane (FIGS. 1a, 1e) or orthogonal thereto (FIGS. 1d, 1f), i.e. symmetrical with respect to the yz-plane. The latter is particularly common when the two coil halves are connected in series.
When the number of feed lines is odd, two feed lines are usually connected to one another outside the coil so that, in operation, the magnitude, for example, of the current in one feed line is equal to the sum of the currents through two feed lines. If the connection is made capacitively and not galvanically, static modes of the coil can be suppressed. If the connection is realized by means of stop filters, couplings with further coils can be reduced. As two feed lines effectively work in the same way as one single, feed lines connected in this way shall also be understood to be “pairs of feed lines”.
The feed lines can be galvanically connected to the coil (FIGS. 1a, 1d, 1f). A capacitive coupling 15 (FIG. 1e) or an inductive coupling is also common in the prior art, particularly in the case of resonators.
When the sample is changed, frequency tuning and impedance matching must be adapted to suit the sample, as the measurement substances and solvents generally have different dielectric constants and losses which change the resonant frequency and impedance of the coil. This change must be corrected by the network.
The simplest tuning variant is a variable capacitor which is connected in parallel with the inductance by means of a pair of feed lines. In doing so, it is most efficient when this inductance constitutes the greatest part of the inductance of the coil. The simplest matching variant is likewise realized by a single variable capacitor which either connects an input port to the network or an input port to ground, such that the port impedance is transformed up or down and matched to the impedance which is applied to the coil feed lines. Alternative forms of impedance and frequency matching include inductances with variable coupling, transmission lines, transformers, capacitive bridges and also couplers such as, for example, quadrature hybrids or rat race or transmission lines as well as their equivalent circuits created by discrete elements.
The effect of a different circuit on the magnetic fields generated by the coil configurations and feed lines is substantially independent of the specific design. The objective with all variants is usually to generate equal and opposite currents and potentials in a pair of feed lines, and therefore the impedances at the measuring frequency or frequencies at the feed lines for different network variants are substantially identical or at least very similar.
In the case of multi-nuclear circuits, it is no trivial matter for all resonant frequencies to achieve the requirement of equal and opposite potentials at the feed lines. Discrepancies between different networks can therefore occur. However, as long as the resonant frequencies are significantly below the Eigen-resonances of the coils used, it can be assumed that the magnitudes of the currents through the feed lines are substantially identical. If the Eigen-resonances are near or even below the operating frequency, it must be ensured that the potentials are adjusted such that the coil can be operated as efficiently as possible. This can be achieved by positioning, the current zeros in operation as far as possible in the feed lines or inside the coil as symmetrically as possible and as close as possible to the feed lines so as to result in a reduction of the B1 field in the sample which is as small as possible.
As a result of changing the tuning and matching elements (generally capacitors or inductances), different currents through the feed lines occur in operation with different samples, which, with nearly all variants of coils/resonators, have a great influence on the flanks of the magnetic field profile of the coils in the test volume. This applies particularly in the case of resonators or coils which are operated close to their Eigen-resonance so that, in operation, the current through the feed lines has a significantly smaller magnitude than the current in the coil configuration.
One objective in the design of a probe head is to generate a high-frequency B1 field profile in the sample which is as rectangular as possible. In particular, signals from regions of the sample which are far away from the central region must be prevented from being detected. The reason for this is that the resolution of an NMR measurement depends on the achievable static B0 magnetic field homogeneity in the measuring volume. The larger this volume the harder it is to achieve a sufficient homogeneity. Particularly problematic is a “flat” HF magnetic field profile for the case of solvent suppression: as shimming the static field usually leads to the edge regions of the detection volume having a slightly different static magnetic field than its central region. As a result of the higher magnitude of the central region of the measured signal, prioritization takes place when shimming, which leads to deviations in the edge region, due to the limited number of available shim functions. Generally, the associated signals carry hardly any weight only with a few or even less than one percent of the total signal and lead to line broadening “at the foot” of the resonance line.
However, as a result of the varying resonant frequency in the edge regions of the samples, the solvent suppression here is insufficient. As the solvent signal can be orders of magnitude larger than the signal of the test substance, the measured spectra have artifacts or broad regions of poorly suppressed solvent signal when a too strong (and frequency-shifted) signal is picked from the edge regions.
FIG. 2a shows a typical magnitude profile of the HF magnetic field of a saddle coil on a log-lin scale. At the top end a zero crossing of the field can be seen, which is masked in the bottom region by the HF magnetic field generated by the currents through the feed lines. This zero crossing comes about as the field lines of the HF magnetic field are closed in themselves. All field lines which pass through the inside of the coil, in particular also through the sample, are closed in space. Some of these field lines are closed in planes parallel to the xy-plane; others, on the other hand, are closed in the xz-plane or arbitrarily. This results in a region in the sample with “reversed” field direction, wherein, as a rule, the amplitude is approximately an order of magnitude less than the field in the center of the coil. In the top region, a limitation of the HF magnetic field to a narrow range can be achieved by skilled positioning of HF screens; in the bottom region, however, this is not possible to the same extent, as only exponential damping of the remaining fields is possible in the HF screen (see FIG. 2c).
In the prior art, the screens in the bottom region can be positioned either directly at or below the level of the bottommost ring elements. As there is no zero crossing of the B field, the slew rate cannot be greater than due to the damping in the HF screen.
The difference between top and bottom region is resolved in that the conductors of the feed lines are spaced apart from one another. This spacing can occur along the periphery (angularly) and also radially. As a result, a B2 field, the orientation of which has an angle α with −90°≤α≤90° with respect to the B1 field of the coil in the central region, is generated between the feed lines.
Equivalent to the B1 field, B2 field is understood to mean the part of the high-frequency magnetic field which occurs as a result of currents through the feed lines to a coil. In order to minimize this field, in the prior art, the feed lines are brought together as closely as possible and, if possible, initially fed radially outwards and only then parallel downwards.
As a high electrical potential difference occurs between the feed lines in operation, limits are placed on the distance of feed lines with different potentials (and opposing currents). If they approach too closely, the dielectric strength of the coil reduces and therefore also the maximum achievable B1 field amplitude in transmit operation. In addition, the capacitance between the feed lines increases, which is often undesirable, particularly with coils and resonators for proton or fluorine detection.
The B1 and B2 fields for the section planes A-A′ and B-B′ defined in FIG. 1a are shown in FIG. 1b. A-A′ is a section plane through the central region of the coil in which the B1 field amplitude reaches at least 50% of the maximum B1 field amplitude in the sample. The plane A-A′ shown in FIG. 1a is coincident with the xy-plane. The plane B-B′ is a plane which intersects the feed lines and is placed along the z-axis below the lowest lying conductor elements of the coil, in particular in a region in which the B1 field of the coil has changed direction.
By definition, within the sample 16, which here is cylindrical in shape, the field lines of the B1 field are oriented substantially parallel to the x-axis (FIG. 1b, top). The orientation of the B1 field is shown schematically in the top diagram in FIG. 1c as a large arrow and forms an angle of 0° with the x-axis. The field lines of the B1 field are closed in themselves so that the orientation of the B1 field in the B-B′ plane has an angle of 180° with the x-axis (not shown).
For the embodiment of the coil according to FIG. 1a, the field lines of the B2 field are, at least on average, likewise aligned parallel to the x-axis over the section B-B′ through the sample while it is also significantly less homogeneous and of lower amplitude than the B1 field in the central region of the coil/sample. In the bottom part of FIG. 1c, the B2 field is likewise shown schematically by an arrow, the orientation of which has an angle of 0° to the x-axis.
A further example is shown in FIGS. 1f to 1 h. For feed lines which are mounted in the direction of the y-axis, a B2 field, which has an angle of 90° (in alternative embodiments also −90°) to the x-axis, occurs in the section plane B-B′. In the case of inhomogeneous fields, the mean direction of the B2 field over the section of the sample in the B-B′ plane must be decisive.
The present invention is based on the object of modifying a nuclear magnetic resonance coil configuration of the kind defined in the introduction with particularly simple technical means such that a high-frequency B field profile, which is as rectangular as possible and is particularly steep on both sides, can be generated, preferably for high-resolution NMR spectroscopy, particularly of liquids.